RR Lyrae stars in Galactic globular clusters

VI. The period-amplitude relation

¹ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: [bono;caputo;dicriscienzo]@mporzio.astro.it
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany 
³ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy

Received: 2 July 2007
Accepted: 18 September 2007

Abstract

Aims.This work uses nonlinear convective models of RR Lyrae stars and evolutionary predictions of low-mass helium burning stellar structures to constrain the properties of cluster and field RR Lyrae variables. In particular, we address two problems: is the Period-Amplitude (PA_V) plane of fundamental (RR_ab) variables a good diagnostic for the metal abundance? Is the M_V(RR)-[Fe/H] relation of field and cluster variables linear over the whole metal abundance range of [ Fe/H2.5 to ~0?

Methods.We perform a detailed comparison between theory and observations for fundamental RR Lyrae variables in the solar neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic globular clusters.

Results.We show that the distribution of cluster RR_ab variables in the PA_V plane depends not only on the metal abundance, but also on the cluster Horizontal Branch (HB) morphology. We find that on average the observed pulsation parameter k_puls connecting the period to the visual amplitude increases when moving from metal-poor to metal-rich GGCs. However, this parameter shows marginal changes among OoI clusters with intermediate to red HB types and iron abundances Fe/H, whereas its value decreases in OoII clusters with the bluer HB morphology, although these clusters are also the less metal-poor ones of the group. Moreover, at [ Fe/H the OoI clusters present redder HB types and larger values than the OoII clusters. The RR_ab variables in ω Cen and in the solar neighborhood further support the evidence that the spread in [Fe/H], at fixed k_puls, is of the order of ±0.5 dex. Using the results of synthetic HB simulations, we show that the PA_V plane can provide accurate cluster distance estimates. We find that the RR_ab variables in OoI and in OoII clusters with very blue HB types obey a well-defined M_V(RR)–k_puls relation, while those in OoII clusters with moderately blue HB types present a zero-point that is ~0.05 mag brighter. Regarding field variables, we show that with [ Fe/H1.0 a unique M_V(RR)–k_puls relation can be adopted, independently of the color distribution of the parent HB star population.

Conclusions. Current findings suggest that the PA_V distribution is not a robust diagnostic for the metal abundance of RR_ab variables. However, the same observables can be used to estimate the absolute magnitude of globular cluster and field RR_ab variables. We show that over the metallicity range the M_V(RR)-[Fe/H] relation is not linear but has a parabolic behavior.